1. Field of the Invention
The invention concerns a method to acquire an MR image with a magnetic resonance system by means of a pulse sequence, a corresponding magnetic resonance system, and a corresponding electronically readable data storage medium.
2. Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in simple terms, the examination subject is positioned in a strong, static, homogeneous basic magnetic field B0 (field strengths from 0.2 to 7 Tesla or more) in a magnetic resonance apparatus so that nuclear spins in the subject orient along the basic magnetic field. For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field. To trigger nuclear magnetic resonances, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject by means of at least one transmission coil, the triggered nuclear magnetic resonances (signals) are measured by reception coils, and MR images (for example) are reconstructed on the basis of the measured signals. The magnetic flux density of these RF pulses is typically designated with B1. The pulse-shaped radio-frequency field is therefore generally also called a B1 field for short. The nuclear spins of the atoms in the examination subject are thereby excited by these radio-frequency pulses such that they are deflected out of their steady state parallel to be basic magnetic field B0 by what is known as an “excitation flip angle” (also called a “flip angle” in the following for short). The nuclear spins then precess around the direction of the basic magnetic field B0. The magnetic resonance signals that are thereby generated are acquired by radio-frequency reception antennas. The measurement data acquired in such a manner are digitized and stored as complex numerical values (also called raw data) in a k-space matrix. By means of a multidimensional Fourier transformation of the values of the k-space matrix, the measurement signals can be calculated to form image data in order to reconstruct an associated MR image from the k-space matrix populated with values. In addition to anatomical images, spectroscopy data, movement data or temperature data of an examined or treated area can be determined with the aid of the magnetic resonance technique.
The manner of the excitation of the spin echoes and their acquisition is controlled by different sequences (known as pulse sequences) that specify the chronological order of the RF pulses and gradients for excitation and spatial coding, as well as the acquisition of the measurement signals.
Depending on the desired contrast, various pulse sequences are used in order to excite and acquire the measurement signals. One known sequence is the (true)FISP sequence (“Fast imaging with steady state precession”). Not only the steady state of the longitudinal magnetization, but also a steady state for the transverse magnetization, is utilized with this sequence. Spin echo components likewise contribute to the image contrast because each excitation pulse also acts as a refocusing pulse. Unfortunately, this sequence construction is prone to artifacts given the use of echo times that are longer than has previously been typical.
Typical, more robust variants of this sequence type are limited to rephasing only the dephasing (due to the phase coding gradients) that are used for the purposes of spatial coding, after the end of the data acquisition.
FISP is only one acronym used for this sequence type. Other known acronyms for this sequence type are GRASS (“Gradient Recalled Acquisition into Steady State”) and FIESTA (“Fast Imaging Employing STeady state Acquisition”) or T2-FFE (“Fast Field Echo, T2-weighted”) or, respectively, bFFE (“balanced Fast Field Echo”).
As imaging methods, sequences of the FISP sequence type have expanded into many fields of application, for example orthopedics, cardiology, colonoscopy, real time imaging etc. They have become a standard sequence for cardio MR (MR imaging of the heart), in particular due to the inherently high blood signal achieved by this sequence type. Particular advantages of this sequence type are a short measurement time per slice, a high signal-to-noise ratio (SNR) in many tissue types and an ideal detail imaging given the use of a conventional k-space scan.
For this sequence type, it is necessary to generate and to maintain the condition known as a “steady state” more precisely a “steady-state free precession” (SSFP) state, i.e. a state in which the magnetization vector is in the equilibrium state. To maintain this state, it is necessary to completely rephase the magnetization at the end of the sequence again at the point in time of the RF excitation pulse. This is achieved by switching (activating) the gradients along all three axes such that the zeroth moment (the integral of the gradient g over time, M0(t):=∫0tg(τ)dτ) is exactly zero at every RF excitation pulse, thus also after a time TR (repetition time) after an RF excitation pulse. If this gradient area described by the zeroth moment is not zero at the point in time of the repetition time TR, this leads to severe image artifacts, known as “banding artifacts”. SSFP sequences that allow the zeroth moment at the repetition times TR to disappear (and therefore are less prone to such artifacts) are also designated as b-SSFP (“balanced steady-state free precession”) sequences.
In the article by O. Bieri and K. Scheffler “Flow Compensation in Balanced SSFP Sequences”, Mag. Res. Med 54, pp. 901-907, 2005, two methods are compared for reducing negative effects (such as signal losses and image artifacts) from a possible non-constant phase development of moving spins during an excitation. One of the methods involves a pairing of successive phase coding steps. The other method uses completely movement-compensated pulse sequences, i.e. pulse sequences whose phase coding gradients are designed such that their first moment M1(t):=∫0tg(τ)·dτ (the integral of the product of gradient g and the time over the time) disappears at every excitation pulse. However, in order to achieve this disappearance, the repetition times TR between two excitation pulses must be extended such that the gradient shapes required for this can be temporally accommodated (see for example FIG. 2 of the article by Bieri and Scheffler).
Methods to generate movement-compensated phase coding gradients are specified in, for example, Chapter 10.4 “Gradient Moment Nulling” (P. 331 through 349) in the “Handbook of MRI Pulse Sequences” by Bernstein, King and Zhou, Elsevier Inc., 2004.